One in six calls to a 24-hour helpline last year concerning mental health came from girls contemplating suicide.
Many more suicidal girls seek help than boys
Some rang ChildLine while attempting to kill themselves, while others had tried to take their own life, and were thinking of doing so again.
In total, more than 6,000 children and young people - some as young as five - called the helpline about mental health problems, ChildLine analysis found.
Problems included depression, eating disorders, bullying and sexual abuse.
On one occasion a ChildLine counsellor helped save the life of a girl who had taken an overdose in her bedroom while her parents were downstairs, unaware of what was happening.
After getting the girl's agreement, the counsellor called the emergency services and an ambulance arrived at the house in time to save the youngster whose grateful father rang the next day to say thank you.
Research suggests rates of depression and anxiety have increased among adolescents in the UK by 70% in the last 25 years.
Joelle Leader, assistant director of ChildLine, said: "When young people talk about suicide they are obviously in deep despair.
"They are at crisis point with no one else to turn to, which is why they call us.
"The number of children who rang to talk about mental health issues last year could have filled 250 classrooms, so it's a big problem."
Ms Leader said ChildLine could offer immediate support and advice, but it was clear that many young people needed more intensive therapeutic support over a sustained period.
"At the moment, there are simply not enough therapeutic services for children with these problems, and we are urging the government to give this issue urgent attention."
Of the calls to ChildLine about suicide (1,265) nearly four out of five came from girls.
However, in the 15 to 21 age group suicide is three times more common among boys.
Eating problems also featured high on the list of mental health concerns, with 1,854 girls and 158 boys ringing for help and advice.
Marjorie Wallace, chief executive of SANE, said the charity had recorded an increase in the number the number of calls to its helpline from young people who self-harmed - sometimes in a brutal fashion.
"Those young people calling about self-harm report an almost doubled rate of suicidal thoughts than non self-harmers, and a significantly higher incidence of past suicide attempts.
"What is alarming is the numbers of those taken to A&E departments who are sent home without any follow-up help.
"We need doctors and teachers to be more alert to the potential risks, and many more therapists available, to prevent the vicious cycle of relief by painful self-harm."